The present invention concerns a flame monitoring apparatus.
Such methods and apparatuses are already known for different purposes and uses. Thus for example DE-OS No. 1 815 968 discloses a flame monitoring apparatus in which an ac voltage is supplied to a transformer and subsequently to a peak voltage limiter. The transmission of voltage peaks from the mains to the operating circuit is prevented by the peak voltage limiter. The voltage limiters used for that purpose are for example voltage-dependent resistors (VDR) which provide a limiting effect in a bipolar mode, that is to say in both voltage directions. A problem of such flame monitoring apparatuses however is rectifier effects at the burner, which are not flame-induced, for example in the case of ionization electrodes due to chemical actions between the monitoring electrode and the reference ground. In limited situations however a flame signal can be simulated by those rectifier effects when a flame is not present. That can result in explosions in the burner installation, and for that reason the attempt is made to avoid the rectifier effects which are not flame-induced, by virtue of sufficient insensitivity in respect of the flame signal amplifiers.
One advantage of the present invention is to make flame monitoring apparatuses of the kind set forth in the opening part of this specification insensitive in relation to non-flame-induced rectifier effects, by suitable measures.
Therefore, one advantage of the invention is that an asymmetric limit voltage which acts on the sensor can be produced.
By virtue of the production of an asymmetric voltage, the negative effects of non-flame-induced rectifier effects with a high alternating current component, as can occur for example due to the deposit of cleaning agents or test sprays between the ionization electrode and the reference ground and for example mains voltages with an unwanted dc voltage offset can be better suppressed. In that way it is possible to avoid unwanted flame signals when no flame is present.
Further advantageous configurations of the invention are set forth in the appendant claims.
If semiconductor devices such as Zener diodes are used to produce an asymmetric voltage, it is possible even to cope with device faults in respect of the Zener diode, due to the higher number of Zener diodes in one direction. If a Zener diode fails there are still sufficient diodes for reliable operation of the voltage limiter. The greater the number of additional Zener diodes that are provided to produce the asymmetry, the correspondingly greater faults it is then possible to compensate.
The structure with Zener diodes does not exhibit any voltage dependency in comparison with varistors (with small series resistors) and temperature compensation can also be implemented by the use of Zener diodes with different temperature coefficients.
If the (unwanted) property of voltage dependency of varistors is to be simulated, that can be done by higher-resistance series resistors in the Zener diode series.
The structure with Zener diodes permits ac voltage stabilization with standard components which can be obtained from a number of manufacturers.
Implementation of ac voltage limitation by means of diodes, for example in the form of a diode section, also affords the advantage that, for example if it may be necessary that the limited ac voltage of an automatic firing device has to be switched over between two voltage values within a switching sequence, a voltage change-over switching operation can be easily implemented by bridging over some diodes of the diode array. In that case the desired voltage variation can be freely selected by way of the choice of the diodes.
In conventional voltage-dependent resistors (VDR) for voltage limitation, the voltage change-over switching procedure would require for example two varistors and a switch or a varistor, a voltage source and a switch.